1. Field of the Invention
The present invention relates to an imaging device, an image reading apparatus, an image forming apparatus, and image reading method.
2. Description of the Related Art
An image reading apparatus reads a document image by irradiating a document by a light source such as an LED, and converting the reflected light from the document to an image signal by an image sensor, and a CMOS image sensor, for example, has been known as one of such image reading apparatuses. There are factors to deteriorate the quality of the image read by the CMOS image sensor, and a known factor is fixed pattern noise that occurs due to variation of each pixel. When the fixed pattern noise is present, a resulting image after reading shall include vertical lines.
The variation of each pixel in the CMOS image sensor includes, for example, variation in dark current of floating diffusion (FD) which functions as an electric charge detecting unit, variation in offset voltage of a pixel transistor, and variation in dark current (current generated even when there is no exposure of light) of a photo diode. Meanwhile, as a method for correcting the fixed pattern noise, a correlation double sampling (CDS) technique for correcting a black level is known by taking a difference between an optical signal level and a reference level including a dark current or the like.
Japanese Laid-open Patent Publication No. 10-126697 discloses a solid state image sensor configured to obtain a reference signal for correcting vertical lines of fixed pattern noise. In the solid state image sensor, a timing generator outputs a substrate pulse immediately before a pixel signal is read, in order to reset a pixel transistor. An operating pulse is then raised quickly in a condition where no light incidence signal is present. Accordingly, a pixel signal is read from the pixel transistor via an operating switch and held in a capacitor, and a horizontal scanning pulse is output from a horizontal shift resistor to electrically connect a horizontal switch one by one in the horizontal image period to output the signal sequentially from the capacitor.
In an area sensor in the past, however, spectral filters of R, G, and B colors are installed for each photo diode in a Bayer arrangement or other type of arrangement. Therefore, the area sensor in the past is unable to read data of multiple colors at the same pixel position, and such data of multiple colors are read at different positions and synthesized in image processing as data of a pixel. Since the area sensor in the past cannot handle the data of multiple colors synthesized for the same pixel position by the same black correction circuits, there has been a problem that a correction error occurs between colors due to the variation of black correction circuits.
Therefore, there is a need to provide an imaging device, an image reading apparatus, an image forming apparatus, and an image reading method that are capable of performing black level correction without generating a correction error between colors at approximately the same pixel position.